The present invention relates generally to ultrasonic surgical devices, and more particularly, to ultrasonic surgical devices useful for removing fibrous, elastic, or tenacious tissue.
Devices that effectively utilize ultrasonic energy for a variety of applications are well known in a number of diverse arts. One of these devices is an ultrasonic horn used for the removal of tissue. The Ampulla or Gaussian profile was published by Kleesattel as early as 1962, and is employed as a basis for many ultrasonic horns in surgical applications including devices described in U.S. Pat. No. 4,063,557 to Wuchinich, et al, 1977, and U.S. Pat. No. 6,214,017 to Stoddard, et al, 2001 for use in ultrasonic aspiration. The Gaussian profile is used in practice to establish and control the resonance and mechanical gain of horns. A resonator, a connecting body, and the horn act together as a three-body system to provide a mechanical gain, which is defined as the ratio of output stroke amplitude of the distal end of the tip to the input amplitude of the resonator. The mechanical gain is the result of the strain induced in the materials of which the resonator, the connecting body, and the ultrasonic horn are composed.
A magnetostrictive transducer coupled with the connecting body functions as a first stage of the booster horn with a mechanical gain of about 2:1, due to the reduction in area ratio of the wall of the complex geometry. The major diameter of the horn transitions to the large diameter of the Gaussian segment in a stepped-horn geometry with a gain of as large as about 5:1, again due to reduction in area ratio. The uniform strain along the length of the Gaussian provides multiplicative gain of typically less than 2:1. Thus, the application of ultrasonically vibrating surgical devices used to fragment and remove unwanted tissue with significant precision and safety has led to the development of a number of valuable surgical procedures. Accordingly, the use of ultrasonic aspirators for the fragmentation and surgical removal of tissue from a body has become known. Initially, the technique of surgical aspiration was applied for the fragmentation and removal of cataract tissue. Later, such techniques were applied with significant success to neurosurgery and other surgical specialties where the application of ultrasonic technology through a handheld device for selectively removing tissue on a layer-by-layer basis with precise control has proven feasible.
Certain devices known in the art characteristically produce continuous vibrations having substantially constant amplitude at a predetermined frequency (i.e. 20-36 kHz). Certain limitations have emerged in attempts to use such devices in a broad spectrum of surgical procedures. For example, the action of a continuously vibrating tip may not have an adequate effect in breaking up certain types of body tissue, more elastic tissue, bone, etc. Because the ultrasonic frequency is limited by the physical characteristics of the handheld device, only the motion available at the tip provides the needed motion to break up a particular tissue. All interaction with the tissue is at the tip, some is purely mechanical, and some is ultrasonic. Some teach in the art that interaction with the tissue at the tip distal and is due only to mechanical interaction. To others, it is clear from experimental results that acoustic power is propagated to the load to aid in tissue fragmentation, emulsification, and aspiration. In any case, the devices have limitations in fragmenting some tissues. The limited focus of such a device may render it ineffective for certain applications due to the vibrations which may be provided by the handheld device. For certain medical procedures, it may be necessary to use multiple hand held devices or it may be necessary to use the same console for powering different handheld devices.
Certain devices known in the art characteristically produce continuous vibrations having substantially constant amplitude at a frequency of about twenty to about fifty-five kHz. Amplitude of transducer-surgical tip systems decreases with increasing frequency because maximum stress in the material of the horns is proportional to amplitude times frequency, and the material must be maintained to an allowed fraction of its yield strength to support rated life in view of material fatigue limits. For example, U.S. Pat. Nos. 4,063,557, 4,223,676 and 4,425,115 disclose devices suitable for the removal of soft tissue which are particularly adapted for removing highly compliant elastic tissue mixed with blood. Such devices are adapted to be continuously operated when the surgeon wishes to fragment and remove tissue, and generally is operated by a foot switch.
A known instrument for the ultrasonic fragmentation of tissue at an operation site and aspiration of the tissue particles and fluid away from the site is the CUSA EXcel® Ultrasonic Surgical Aspirator (Integra LifeSciences Corporation, Plainsboro, N.J.). When the longitudinally vibrating tip in such an aspirator is brought into contact with tissue, it gently, selectively, and precisely fragments and removes the tissue. The CUSA® transducer amplitude can be adjusted independently of the frequency and this amplitude can be maintained under load depending on reserve power of the transducer. In simple harmonic motion devices, the frequency is independent of amplitude. Advantages of this unique surgical instrument include minimal damage to healthy tissue in a tumor removal procedure, skeletoning of blood vessels, prompt healing of tissue, minimal heating or tearing of margins of surrounding tissue, minimal pulling of healthy tissue, and excellent tactile feedback for selectively controlled tissue fragmentation and removal.
In an apparatus that fragments tissue by the ultrasonic vibration of a tool tip, efficiency of energy utilization is optimized when the transducer which provides the ultrasonic vibration operates at resonant frequency. The transducer and surgical tip design establishes the resonant frequency of the system, while the generator tracks the resonant frequency and produces the electrical driving signal to vibrate the transducer at the resonant frequency. However, changes in operational parameters, such as changes in temperature, thermal expansion, and load impedance, result in deviations in the resonant frequency. Accordingly, controlled changes in the frequency of the driving signal are required to track the resonant frequency. This is controlled automatically in the generator.
Conventional ultrasonic surgical aspirating tips employed in surgery for many years typically present a longitudinally vibrating annular surface with a central channel providing suction or aspiration, which contacts tissue and enables fragmentation via described mechanisms of mechanical impact (momentum), cavitation, and ultrasound propagation. Mechanical impact may be most useful in soft tissue and cavitation clearly contributes to the fragmentation of tenacious and hard tissue in situations where liquids are present and high intensity ultrasound exceeds the cavitation threshold.
Ultrasound propagation is concerned with the transmission of pressure across the boundary of a surgical tip and tissue, which leads to the propagation of pressure and, perhaps more importantly, particle displacement. Acoustic impedance is the total reaction of a medium to acoustic transmission through it, represented by the complex ratio of the pressure to the effective flux, that is, particle velocity times surface area through the medium. As discussed in the classic text of Krautkramer J. and Krautkramer H, ULTRASONIC TESTING OF MATERIALS, Berlin, Heidelberg, N.Y., 1983, for the case of a low to high acoustic impedance boundary, it may seem paradoxical that pressure transmitted can exceed 100%, but that is what results from the build-up of pressure from a low to high acoustic impedance boundary. In the case of a high to low acoustic impedance mismatch, such as with a high impedance titanium ultrasonic horn to low impedance fibrous muscle, soft tissue, or water, the pressure transmitted decreases (e.g., less than 15% for titanium to fibrous muscle) and particle displacement increases (e.g., as great as 186% for titanium to muscle).
Conventional ultrasonic surgical aspirating tips have been found to be efficient in the removal of soft tissue, and with emergent bone tips, applicable to the removal of hard tissue; however, some fibrous, elastic, and tenacious tissues persist in difficulty of removal. It has been found that using such conventional ultrasonic horns and devices that employ only the effects of intensification of ultrasound or sharpened edges to remove bovine fibrous muscle tissue, leaves a fibrous elastic skeleton. Thus, there remains a need for ultrasonic surgical devices with innovative aspiration tips that allow for more effective removal of fibrous tissue via the enhanced utilization of ultrasound fragmentation effects.
It is known that materials often fail, fracture, tear, or rupture, more readily as a result of a shear force rather than in tension or compression. Common examples include paper, garden bushes, hair, cloth, steel shear bolts or pins, and collagenous materials. A thin fibrous sheet of paper can be pulled or snapped with a greater tension force, but it can much more easily be ripped by the fingers applying light forces in opposite directions (shear). Likewise, scissors readily cut paper by employing a shear force concentrated by opposing edges of the scissors. Studies of mechanical behavior of materials have shown that biologic tissue is viscoelastic material, meaning that it has a time-dependent stress-strain relationship. The effect of the strain rate on the material is critical to causing fragmentation. The ultrasonic horn of the present invention evolved from imagining innovative ways of introducing scissor or shear ultrasonic effects with a surgical aspirating tip.
Hence, those skilled in the art have recognized a need for an ultrasonic aspiration tip that allows more effective removal of fibrous, elastic, and tenacious tissues. The present invention fulfills this need and others.